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Variation in the Three-spine Stickleback
Text & Images by Paul Barrow
First published in the Red & Green
Click on images for bigger view

On the subject of regional differences in native fish, the most widespread, the three-spined Stickleback, is also the most variable. The extremes running from naked bodied fish with no spines to a rumoured giant variety with 4 dorsal spines living in the deep, drowned structures of Keilder Water.
The amount of armour present, side scutes and spines, is directly linked to predation: the least armoured coming from Scottish islands without predators and the most heavily armoured from places such as the dangerous world of the saltwater estuary. 
Three sub-species of the Three-spined Stickleback occur in Europe: Gasterostreus aculeatus leiurus, Gasterostreus aculeatus semiarmatus and Gasterostreus aculeatus trachus.
These differ in the amount of armouring and spines present, but the most common variety has 3 dorsal spines, 2 pelvic spines and a single anal spine. 
The dorsal and pelvic spines are protected from collapse by a pelvic girdle (left) that connects them in a strong, crush-proof structure surrounding the body – similar to Corydoras. All sub-species share a common body shape: the dorsal and ventral contours are curved and (usually) evenly matched, the dorsal and anal fins are set well back along the body close to the long, thin caudal peduncle, and each pelvic fin is reduced to one spine.


Females & non-breeding males: colour varies due to location and conditions. Freshwater fish can be olive green above, fading to bronze or silver below; an overall bronze with grey mottling above; an overall bronze/green or a bright, brassy colour. 
Males: during the breeding season males develop a red breast and more colour above as the one on the left.


Sub-species variations.
Gasterostreus aculeatus leiurus: the most lightly armoured variety, has no, or very few, body plates, one to three dorsal spines, pelvic spines may or may not be present. Mainly found in freshwater.

Gasterostreus aculeatus trachus. The most heavily armoured, the body is fully plated and all spines are present. This variety is found in salt water, estuaries and is the most widespread freshwater sub-species in Eastern Europe.

Gasterostreus aculeatus semiarmatus: this is intermediate between the two above having a partly plated body.